A radio communications system is comprised, at minimum, of a transmitter and a receiver. The transmitter and the receiver are interconnected by a radio-frequency channel to permit transmission of an information signal therebetween. A digital receiver will generally include an amplifier with a gain adjusted by a control signal. The process of adjusting the gain of a received signal using a control signal is called Automatic Gain Control (AGC).
Although the concept of AGC in radio signal reception is well understood, automatic gain control of Time Division Multiple Access (TDMA) signals presents new challenges to the land-mobile industry.
Increased usage of cellular communications systems has resulted, in many instances, in the full utilization of every available transmission channel of the frequency band allocated for cellular radio telephone communication. In wide band TDMA systems, such as the cellular system proposed for use in the United States, hereinafter called USDC for United States Digital Cellular, and RF channel is shared (time-division-multiplexed) among numerous subscribers attempting to access the radio system in certain ones of various time-division-multiplexed time slots. This permits transmission of more than one signal at the same frequency, using the sequential time-sharing of a single channel by several radio telephones. The time slots are arranged into periodically repeating frames thus, a radio communication of interest may be periodically discontinuous wherein unrelated signals are interleaved with signals transmitted in other time slots. The unrelated signals (of widely varying strength) must not influence the gain control of the signals of interest. Varying signal strength may be caused by distance loss or multipath fading. A formidable challenge then is to provide Automatic Gain Control of these periodically discontinuous TDMA signals.
The challenge is further enhanced by attemping to provide digital AGC in inexpensive receivers, specifically, those having Analog-to-Digital converters (A/D) with limited dynamic range. Since the received signal strength may vary by as much as 120dB in the hand-mobile environment, but 8-bit Analog-to-Digital (A/D) converters, moderately priced, for digital signal processing are limited to 48dB dynamic range, techniques must be developed for controlling the gain of the portions of the radio receiver prior to the A/D converters to keep the signal at the input to the A/D converters within the limited dynamic range of the A/D converters. The challenge then is to handle a 120dB range discontinuous signal with a 48dB device; otherwise, prohibitively expensive A/Ds with greater dynamic range must be utilized.
Certain operations necessary for new communications system protocols require the subscriber unit to tune its receiver to another channel, measure signal strength, and report the measured signal strength to the system. This operation provides the system with information about which channel has the strongest signal and hence can offer the best level of service to the subscriber unit. The system may then direct the unit to change the channel with which it communicates with the system. This process is called Mobile Assisted Handoff, or MAHO.
This operation provides a very limited time for measuring the power of the to-be-tested channel, since the testing operation must be done during idle periods of the discontinuous signalling. Therefore, a fast AGC response time is needed to normalize the received signal strength energy level within the range of the A/D converters. Conventional AGC control loops are difficult to implement with rapid and accurate response times. This is because AGC control loops, conventionally implemented, include the channel selectivity filters or matched filters in the control loop. These filters have multiple poles and possibly zeros, hence the gain control signal must be filtered additionally by a low-pass filter with a much lower cutoff frequency to insure that the loop response is stable. This low-pass filter, for a conventional AGC system, has a slow response time, and hence limits the response time of the AGC loop.
Additional A/D converter and signal processing methods have delay which also adds to the loop's response time and instability. Conventional operation for AGC function using a signal processor requires a periodic calculation cycle, at the end of which a new gain control signal is generated which replaces the present gain control signal. This periodic calculation cycle is an artifact of the discrete-time nature of signal processors.
Since the received signal energy level is measured after the receiver's base band filtering and the A/D converters, the average signal energy level measured over the span of time since the last calculation cycle was performed and the last gain control signal was generated, will include both energy level measurements generated by the receiver response to the present gain control signal, as well as energy level measurements generated by the receiver response to the previous gain control signal, Thus, when a new gain control signal is issued by the signal processing element, a period of time elapses before the signals present at the inputs to the signal processing element begin to respond to those changes, and a further period of time elapses before the response to the new gain control signal has completed.
The energy level effects remaining from the previous received signal, received during present calculation cycle span, but in response to the previous control signal, result in an inaccurate estimate of the present received signal energy level, and hence potential instability in the AGC control loop.
The AGC problem is further complicated by the fact that the input signal level multiplies the gain of the control loop, hence, when a gain error due to change in received signal level occurs, the loop response time is affected by how much gain error is present.
Yet another challenge for automatic gain control is introduced by variation in gain of the receiver signal amplifiers, and in effective delay through receiver filters, due to temperatures and manufacturing tolerances. For a conventional AGC method, further restriction of the loop's response time must be made to assure stability with variations in gain and filter characteristics.
This invention takes as its initiative to overcome these challenges and realize certain advantages, presented below.